Annually, underground mining of valuable materials is the cause of numerous injuries to and deaths of mine personnel. Governments worldwide have enacted restrictive and wide-ranging regulations to protect the safety of mine personnel. The resulting measures required to comply with the regulations have been a contributing cause of significant increases in underground mining costs. Further increases in mining costs are attributable to global increases in labor costs generally. Increases in mining costs have caused numerous low grade deposits to be uneconomic to mine and therefore caused high rates of inflation in consumer products.
To reduce mining costs and provide for increased personnel safety, a vast amount of research has been performed to develop a mining machine that can excavate materials continuously and remotely. Although success has been realized in developing machines to mine materials continuously in soft deposits, such as coal, soda ash, talc, and other sedimentary materials, there continue to be problems in developing a machine to mine materials continuously in hard deposits, such as igneous and metamorphic materials. As used herein, “soft rock” refers to in situ material having an unconfined compressive strength of no more than about 100 MPa (14,000 psi) and a tensile strength of no more than about 13.0 MPa (2,383 psi) while “hard rock” refers to in situ material having an unconfined compressive strength of at least about 150 MPa (21,750 psi) and a tensile strength of at least about 15 MPa (2,750 psi). Ongoing obstacles to developing a commercially acceptable continuous mining machine for hard materials include the difficulties of balancing machine weight, size, and power consumption against the need to impart sufficient force to the cutting device to cut rock effectively while substantially minimizing dilution, maintaining machine capital and operating costs at acceptable levels, and designing a machine having a high level of operator safety.
For example, a common excavator design for excavating hard rock is an articulated excavator having a rotating boom manipulated by thrust cylinders and an unpowered cutting head having passive cutting devices, such as a box-type cutter using discs or button cutters. Such excavators typically only impart 25% of the available power into actual cutting of the rock and can be highly inefficient. Unproductive parts of the cutting cycle are substantial. For example, repositioning of the excavator requires some actuators to be extended and others retracted until a desired position is reached at which point the extended actuators are retracted and the retracted actuators extended. During excavator repositioning, no excavation occurs.